Salads and vegetables in UK supermarkets are grown by migrant farmworkers who are underpaid, treated like slaves, forced to subsist in filthy conditions and regularly develop health problems associated with pesticides, an investigation suggests.

The deplorable working conditions were uncovered by a Channel 4 News investigation due to be broadcast on Wednesday evening.

An army of migrant workers who produce vegetables exported to the UK said they are forced to work in unsanitary conditions and must use bushes close to where vegetables grow as a toilet. In the event nature calls, they said they are forced to do without water or soap to wash their hands.